Fix error ocurring when displaying storage with null storage type

This is to address the following error:

[NoMethodError] undefined method `downcase' for nil:NilClass
/var/www/miq/vmdb/app/views/layouts/quadicon/_storage.html.erb:5

https://bugzilla.redhat.com/show_bug.cgi?id=1136321
Merge pull request #580 from mzazrivec/none_storage_type_display_fix

Fix error ocurring when displaying storage with null storage type